Field Of The Invention
The present invention relates to an intumescent composition which forms an intumescent-ablator coating which provides an increased level of thermal protection to substrates upon which it is coated. More particularly, the present invention relates to an intumescent coating which is tough and environmentally stable, which yields an insulative char and which contains inorganic fillers which provide endothermically decomposing products.
Background Of The Invention
Intumescent coating compositions in the past have been prepared by combining an intumescent agent with a suitable polymeric binder system. When such a coating is heated to the temperature at which the agent intumesces, the coating expands to many times its original volume thereby forming a charred layer which provides a protective barrier from flame and heat. Among the conventional intumescent compositions are those which contain salts of nitroaromatic amines such as the ammonium salt of 4-nitroaniline-2-sulfonic acid and 4,4'-nitrosulfanilide. Another series of intumescent coating compositions are those which contain phosphate derivatives as intumescent agents. These derivatives form an acid which reacts with a carbon containing material. A low melting resin is required as a binder to allow the special blowing agents to release gases at the decomposition temperature when the coating is exposed to heat or fire. All of the conventional intumescent compositions require heat or fire to activate the intumescent process. In the case of the nitroaromatic amine intumescent agents, this process occurs at relatively high temperature and is highly exothermic because of the nature of the reactions, a rapid rise in temperature of from room temperature to about 400.degree. C occurs in the uncoated or backface side of the coated substrate. This rapid temperature rise provides, without a specific temperature level, times of only one to five minutes control and thus is detrimental from the viewpoint of low temperature, short time, duration protection of the coated substrate. Moreover, some of the prior art intumescent compositions, notably those based on ammonium phosphate and salts of nitroaromatic amines such as p-nitroaniline bisulfate and the ammonium salt of 1,4-nitroaniline-2-sulfonic acid possess environmental stability problems and thus only afford limited environmental protection.
Several techniques have been developed in the past for protecting various substrates from the ravages of heat. One technique involves the incorporation of a sublimation agent in paints which are coated on a substrate (R. Feldman, ACS Organic Coatings and Plastics Chemistry Preprints, 33 (1), 1973). These fire retardant paints, however, are essentially non-intumescing materials which form a porous layer of debris by the release of cooling gases during sublimation. Another prior art fire retardant formulation employs an endothermic compound such as alumina hydrate which diminishes smoke generation as well as reduces flammability by absorbing the heat from the source of fire.
A need, therefore, continues to exist for a method by which the rapid temperature rise caused by the onset of exothermic intumescense in an intumescent coating on a substrate can be reduced or eliminated.